This invention concerns electrical circuit breakers of the kind comprising, for the or each pole thereof, a fixed contact, a movable contact arm carrying a movable contact and swingable between an `on` position in which the movable contact engages with the fixed contact, and an `off` position in which the movable contact is separated from the fixed contact, the circuit breaker further including a tripping mechanism which serves, upon passage through the circuit breaker of current in excess of the rated current of the breaker, to cause the movable contact arm or arms to swing into the `off` position.
The invention is more particularly concerned with circuit breakers of this kind (hereinafter called "of the kind described") in which the movable contact arm is spring loaded towards the fixed contact so as to be capable of limited movement against the action of the spring and away from the fixed contact independently of the tripping mechanism, such as arrangement permitting the movable contact arm to move or be "blown back" under the influence of the very substantial electromagnetic forces which arise in the event of passage of short circuit current, so that contact opening movement of the movable contact arm is initiated upon the occurrence of a short circuit even before operation of the tripping mechanism. This spring-loading of the moving contact arm, permitting a limited amount of independent movement of the arm under the electromagnetic blow-back forces arising on passage of short-circuit currents, contributes to protecting the components of the circuit controlled by the circuit breaker by restricting the length of time during which any short circuit current may pass therethrough.
It is well known, in electrical circuit breakers, to provide, adjacent the movable contact arm, a series of so-called "arc-quenching plates", these each being of V-shaped configuration and arranged approximately in an arcuate array so that the swinging movement of the movable contact arm causes the movable contact to move through the successive recesses defined by the limbs of the successive arc-quenching plates.
As hitherto proposed, the shapes and dimensions of such arc-quenching plates have generally been such that each said plate defines a generally V-shaped recess for passage therethrough of just the free end of the movable contact arm and the movable contact carried thereby. Thus, as a general rule, the arc plates in the prior known constructions have not contributed significantly to the generation of blow-back forces upon the occurrence of a short-circuit.
The present invention is based upon the appreciation that said arc-quenching plates can be utilised to improve the blow back forces occurring upon passage of short circuit currents, thereby to assure a more rapid contact separation, with consequential improved protection for the circuit breaker and the circuit controlled thereby.